In a fuel cell stack for hydrogen-powered fuel cell vehicles, since a fuel cell stack has to be sealed against reaction gases (e.g., hydrogen/air or oxygen) and cooling water (e.g., coolant), each unit cell should have a gasket. Therefore, the gasket for a fuel cell stack requires having high elasticity and substantially low compression set in a suitable range of hardness and exhibit excellent mechanical properties, acid resistance, hydrolysis resistance, heat resistance and electrical insulation properties. In addition, the gasket must exhibit low diffusivity and permeability for reaction gases and cooling water, include none or a small amount of impurities causing catalyst poisoning to achieve high productivity and reduce a production cost.
Generally, for a rubber gasket for fuel cell stacks, fluoroelastomers, silicone elastomers and hydrocarbon elastomer are widely used. Fluoroelastomers are classified into FKM, FFKM and the like, according to American Society for Testing and Materials (ASTM) standards. Fluoroelastomers are attractive material since they show excellent elasticity, acid resistance, heat resistance and the like, and thus are stably used for a substantial period of time under the operating conditions of a hydrogen-powered fuel cell vehicle. However, fluoroelastomers are disadvantageous since they are poor in injection-moldability and cold resistance and are expensive, thus mass production thereof is restricted. Meanwhile, when the fluoroelastomer having excellent basic properties such as elasticity, heat resistance and the like, is crosslinked with peroxide, production cost increases, but airtightness may be secured even at a substantially low temperature of about −30° C. or lower.
However, since a fuel cell stack for fuel cell vehicles includes several hundreds of unit cells and each unit cell is provided with a gasket, the fluoroelastomer is not desirable since a substantial amount of expensive fluoroelastomer needs to be used for the purpose of improving cold resistance. Furthermore, the fluoroelastomer barely has elasticity at a temperature as low as about −40° C., although the fluoroelastomer has excellent cold resistance.
Silicone elastomers are classified into general silicone rubbers, for example, polydimethylsiloxane and the like; and modified silicon rubbers, for example, fluorosilicone and the like. Although solid-type silicone rubbers may be used, liquid-type silicone rubbers may be more frequently used because it is advantageous to precise injection molding. Generally, the liquid-type silicone rubbers have an advantage of exhibiting excellent injection moldability, but have a disadvantage by eluting silicone as an impurity and further cause poisoning of platinum catalyst and reducing fuel cell performance. Thus, they are not suitable for fuel cell applications.
Among hydrocarbon elastomers, ethylene-propylene diene monomer (EPDM) rubber, ethylene-propylene rubber (EPR), isoprene rubber (IR), isobutylene-isoprene rubber (IIR) and the like are frequently used. These hydrocarbon elastomers are advantageous due to their excellent airtightness even at a substantially low temperature of about −40° C. or lower and low cost. However, the hydrocarbon elastomers are disadvantageous in that they cannot be easily used at a high temperature of about 120° C. or higher due to insufficient mechanical properties and heat resistance. For example, their physical properties such as elasticity, oxidation resistance and the like are deteriorated at substantially high temperatures.
Generally, a conventional gasket for fuel cells is integrated with a membrane-electrode assembly, a gas diffusion layer, a separator or a polymer frame by an injection molding process, and is made of any one from a fluoroelastomer, a hydrocarbon elastomer and a silicone elastomer. However, as described above, a gasket made of a single material cannot secure long-term use stability at both substantially low temperatures and substantially high temperatures, so a gasket made of two or more kinds of materials has been considered.
For example, a gasket for fuel cells, which is integrated with a component of a fuel cell such as a separator or an electrolyte membrane, has been developed. In such gasket for fuel cells, a rubber material having substantially low gas permeability is used for sealing gas passage portions, and a rubber material having substantially high gas permeability is used for sealing cooling water passage portions. However, the manufacturing process may be complicated since two types of gasket materials are integrated with any one component of a fuel cell. Moreover, since each of the gasket materials has certain optimal molding and crosslinking condition, the two types of gasket materials do not sufficiently exhibit desired physical properties when they are prepared under the same molding and crosslinking conditions.
In another example, a gasket for fuel cells, which is integrated with a component of a fuel cell such as a separator, a gas diffusion layer or a membrane electrode assembly, has been developed. The gasket therein is made by combining at least two or more types of rubber or resinous materials, therefore resinous materials, therefore the gasket includes a first layer attached to the component and a second layer covering the first layer. However, since the different gasket materials of two types are integrated with any one component of a fuel cell, the interlayer of interfacial adhesion in the gasket may be problematic, especially, when the second layer is formed on the first layer by an injection molding process. In such case, since the flowability of a gasket material for the second layer on the surface of the first layer is low, it may be difficult to obtain satisfactory molded products and the two types of gasket materials may not sufficiently exhibit desired physical properties under the same molding and crosslinking conditions.
Meanwhile, in order to solve the problems occurring when two types of gasket materials are independently prepared and used together, a technique to blend different types of rubber materials using a crosslinking agent has been attempted. So far, in the manufacture of a blended rubber gasket for fuel cells, technologies of blending fluorocarbon rubber and silicone rubber have been generally used. However, technologies of blending fluorocarbon rubber and hydrocarbon rubber have hardly been developed yet.
It is to be understood that the foregoing description is provided to merely aid the understanding of the present invention, and does not mean that the present invention falls under the purview of the related art which was already known to those skilled in the art.